Hassan Khaleghi

Grade:  Master

Thesis Title:

A Constitutive Damage Model for Time-dependent Behavior of Thermoplastic Polymers

Year: Sept. 2018 - Oct. 2021.

Abstract:

In this research, a damage model based on thermodynamic laws for failure prediction of thermoplastic polymers is presented. The proposed model is based on the Eindhoven glassy polymer model (EGP), which is a phenomenological model to predict the mechanical behavior of polymers. The EGP model can well describe polymers’ viscoelastic and viscoplastic behavior, including the dependence of the yield stress on the strain rate and the intrinsic strain softening. The damage parameter is introduced to the model with a continuous damage mechanics approach and the concept of effective stress. In the damage evolution law, the equivalent plastic strain and the hydrostatic stress influence the damage, and with increasing hydrostatic stress, the material’s behavior becomes more brittle. This model is implemented as a constitutive model using the UMAT subroutine in the ABAQUS package. The material’s behavior under uniaxial, biaxial, and triaxial loading for an element with an integration point is investigated. Then, a tensile bar is modeled in the ABAQUS to consider the actual test conditions and the necking. The stress, damage, and equivalent plastic strain distribution in this geometry are investigated. The results show that with increasing strain rate, the damage evolution also increases. Next, a T-fitting pressure test is simulated to show the model’s capabilities in predicting failure in complex loadings and geometries. Burst pressure and the failure location predicted by this model exhibit a good agreement with the experimental results.

Keywords: Continuum Damage Mechanics, Thermoplastic Polymers, Eindhoven Glassy Polymer,

Viscoelastic-viscoplastic behavior.